Lithium ion batteries and other lithium secondary batteries have become increasingly important in recent years as vehicular power sources and as power sources for personal computers and mobile and portable electronic devices. In particular, lithium secondary batteries, because they provide a high energy density at a low weight, are preferentially used as high-output power sources for installation in vehicles. One implementation of this battery is the hermetically sealed lithium secondary battery. This battery is typically fabricated by introducing an electrode assembly—itself having positive and negative electrodes that are provided with active materials—into a battery case together with an electrolyte (typically an electrolyte solution) and then sealing the opening (hermetically sealing).
Hermetically sealed lithium secondary batteries are generally used in a controlled state that keeps the voltage in a prescribed range (for example, 3.0 V to 4.2 V); however, they may become overcharged when the prescribed voltage is exceeded when a greater than normal current is supplied due to, for example, malfunctions. When overcharging occurs, gas may be generated due to decomposition of the electrolyte and/or the temperature within the battery may rise due to heat generation by the active material. A current-interrupt mechanism—which interrupts the charging current through the operation of a current-interrupt valve when the pressure within the battery case meets or exceeds a prescribed value due to, for example, gas generation—is therefore widely used in order to deal with this overcharging.
A known technique when such a current-interrupt mechanism is employed is the preliminary incorporation in the electrolyte of a compound that has a lower oxidation potential (i.e., the voltage at which oxidative decomposition begins) than the nonaqueous solvent of the electrolyte (such a compound is also referred to herebelow as an “overcharge inhibitor”). When the battery enters an overcharged state, the overcharge inhibitor undergoes oxidative decomposition—prior to the occurrence of electrolyte decomposition—and a large amount of gas is generated as a result. This gas causes the internal pressure in the battery to undergo a rapid rise and can thus induce an earlier (i.e., the battery is in a safer state) operation of the current-interrupt mechanism. Aromatic compounds, e.g., cyclohexylbenzene (CHB) and so forth, are a typical example of this type of overcharge inhibitor.
In order to induce an even quicker operation of this mechanism, a technique has recently been proposed in which the amount of gas generation is increased through the addition of an inorganic compound to the positive electrode mixture layer. Patent Literature 1 and Patent Literature 2 are examples of this type of conventional art. Patent Literature 1 states that the reaction efficiency of the overcharge inhibitor can be raised by the addition to the positive electrode mixture layer of a phosphate salt (a compound that contains the phosphate ion) acting as a reaction catalyst. Patent Literature 2 states that, when a carbonate salt (specifically lithium carbonate) is incorporated in the positive electrode mixture layer, this carbonate salt undergoes decomposition when overcharging occurs and a large amount of carbon dioxide gas can be generated as a result.